First off, each mod should have individual instructions as to how to install it. As a general rule, mods go in the C:\Program Files\Stardock\TotalGaming\GalCiv2\Mods, \GalCiv2\DarkAvatar\Mods or \GalCiv2\Twilight\Mods folder, depending on what version of the game it is for. After that, you launch the game, go to options, in the mod section, click 'change', select your mod, then restart the game. If it is the first time playing mods, you probably should click the 'default' button on the mod screen, then restart and try again.
Hope this helps
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